In general, an oxide film is used as a material buried for forming an element isolation insulating film that partitions a field effect transistor. It is known that, as the channel width of the field effect transistor becomes smaller, there is more easily caused a narrow channel effect in which the threshold voltage increases or decreases. This is thought to be caused by migration of boron (B) from the channel region of the field effect transistor into the oxide film composing the element isolation insulating film, and/or by the presence of a fixed charge contained in the oxide film. On the other hand, the oxide film composing the element isolation insulating film generates a film stress depending on the history of a heating process, and thereby brings about a change in the channel mobility of the field effect transistor. Accordingly, it is desired to reduce fluctuations in the transistor characteristics caused by the element isolation insulating film as described above.